TLS support

[jimpo]

What is the best interface for TLS-enabled variants of manet.Dial and manet.Listen? In the Go stdlib, the crypto/tls package implements functions:

func Dial(network, addr string, config *Config) (*Conn, error)
func Listen(network, laddr string, config *Config) (net.Listener, error)

The net/http package sets the precedent of appending "TLS" to the end of TLS-enabled function names with http.ListenAndServeTLS.

How do you feel about adding to go-multiaddr-net the following functions?

func DialTLS(remote ma.Multiaddr, config *tls.Config) (*Conn, error)
func ListenTLS(laddr ma.Multiaddr, config *tls.Config) (Listener, error)

Another option would be to include the TLS information in the multiaddr struct, which I posted about here: multiformats/multiaddr#47.

[Stebalien]

I'd rather not make this package depend on TLS, go is really bad about trimming deps it doesn't need. Maybe this deserves a new go-multiaddr-tls package (if someone really needs it)?